public class CandidateKey {

	// This method acts as a uniqueness checker for the Candidate object based on the Email__c field
	public static void hasCandidateDuplicates(Candidate__c[] candidates){
	
        	//TODO: Define a map for unique Candidates called "candidateMap" that maps a String to the Candidate
         Map<String, Candidate__C> candidateMap = new Map<String, Candidate__C>();
		//TODO: Declare a for loop to cycle through the candidates.
         for (Candidate__c Candidate:candidates){
         
            		if (candidate.email__c != null){
	        		String lastName = candidate.Last_Name__c.toLowerCase();
	        		String email = candidate.Email__c.toLowerCase();
	        		String uKey = lastName + email;       	        	
             
            			//look for dupes within the batch
				    if(candidateMap.containsKey(uKey)) candidate.addError('Duplicate LastName + Email found in batch'); 
				    else candidateMap.put(uKey, candidate);      
            		}
         }        
            // TODO:  Within that loop, check to verify that the email is not null and if it has a value, store the lastName+email in a String called "uKey"
			// and then check to see if there are duplicate uKeys within this batch of records.  If there are, use addError to prevent them,
			// and if not, add the uKey/candidate combo to the candidateMap.
		
  		//Now hit the DB looking for dupes
  		if (!candidateMap.isEmpty()){
	  		for(Candidate__c[] candidatesCheck:[select Id, unique_key__c from Candidate__c where unique_key__c IN :candidateMap.keySet()]) {
				for(Candidate__c candidate:candidatesCheck) {
	  				String currentCandidateId = candidate.Id;
	  				//if there's a dupe and this isn't the current record, then add the error.
	  				if(candidateMap.containsKey(candidate.unique_key__c) && (currentCandidateId != candidateMap.get(candidate.unique_key__c).Id)) {
	     				candidateMap.get(candidate.unique_key__c).addError('Duplicate LastName + Email found in salesforce(id: ' + candidate.id + ')');
	  				}
	  			}
	    		}
  		} 
  	} 
}